1. Field of the Invention
The present invention relates to a piezoelectric resonator device defined by a plurality of piezoelectric plates polarized approximately in the same direction and assembled together in a laminated structure.
2. Description of the Prior Art
A piezoelectric resonator device with a laminated structure is known to vibrate in many different ways. For example, the vibrations may take place in the plane of the plate or in the thickness direction. The vibrations taking place in the plane of the plate are referred to as plane mode vibrations, which are also known as contour mode, expansion mode, lengthwise mode, widthwise mode, radial mode, and area mode. The vibrations taking place in the thickness direction are referred to as thickness mode vibrations.
When operating the piezoelectric resonator device under the plane mode vibrations, it also accompanies thickness mode vibrations as spurious responses, which are unwanted vibrations. Therefore, in order to obtain a preferable operation of the piezoelectric resonator device, it is necessary to eliminate or suppress the thickness mode vibrations.
To this end, according to the prior art, various methods are proposed. For example, in the case of a piezoelectric resonator device defined by a rectangular piezoelectric plate operated under expansion mode, the unwanted thickness mode vibrations (the spurious responses) are suppressed: (i) by the employment of a recess at the center of each side of the piezoelectric plate; or (ii) by the deposition of a mass, such as a solder layer, on an electrode formed on the piezoelectric plate.
But, the method (i) has such disadvantages that the employment of recesses results in reduction of strength characteristics and that the degree of suppress is very small. Also, the method (ii) has such a disadvantage that a pressure contact between the electrode deposited with the solder bead and a terminal member touching on this electrode becomes unstable such that the contact condition varies after a long term by the oxidization of solder bead, resulting in poor electric contact therebetween.